1. Field of the Invention
This invention relates to the stabilization of hydrolyzed silicic acid esters. More particularly, this invention relates to a method of lengthening the shelf life of completely or partially hydrolyzed silicic acid esters. This invention further relates to stabilized hydrolyzed silicic acid ester compositions.
2. Discussion of the Prior Art
It is known to use hydrolyzed preparations of silicic acid esters as binding agents for ceramic articles and for zinc dust pigments. In practice, the preparation of these hydrolyzed preparations usually begins with tetraethylorthosilicate or a condensed ethylsilicate having a silicon dioxide content of 40%. The hydrolysis of these starting products is performed by partial or complete reaction of the ester groupings with water, usually using an acid catalyst, such as an aqueous 0.5 to 2% HCl solution, the starting products being diluted with anhydrous solvents such as C.sub.1 to C.sub.3 alcohols, ketoalcohols with up to 6 carbon atoms, C.sub.3 and C.sub.4 ketones, or methyl acetate.
The above results in the formation of mostly silanols with free OH groups present partially in the form of chains composed of ##EQU1## building blocks. In these building blocks, of which an average of 2 to 10 are present per molecule, the free valences of the silicon atom are bound by hydroxyl groups or unreacted alkoxy groups. It is through this hydrolysis that it becomes possible for the silicic acid ester to be used as a binding agent. The binding agents are hardened by drying or the action of hardening accelerators of a usually basic reaction.
Among other uses, these hydrolyzed silicic acid esters serve as binding agents in the preparation of investment molds based on "lost" models. In this process a shell mold of ceramic material is built up by repeatedly coating a model made of wax or plastic or other such material with a suspension of a finely divided refractory material in a hydrolyzed silicic acid ester preparation. After the ceramic shell has hardened the model is removed and the resultant casting mold is fired. It is especially suited for the manufacture of very precise castings having a fine surface.
Hydrolyzed preparations of silicic acid esters are also used in the manufacture of massive divided ceramic casting molds. In this process, the molding composition consists of a refractory material containing even coarser grains, and a silicic acid ester preparation. A hardening agent is added to this molding composition in such quantity that, after the molding composition has been poured over a divided model, it will harden within a set time. After the molding composition has hardened the divided model is removed, and the resultant mold is fired. Castings made in such molds have a smooth surface and are of great accuracy.
When hydrolyzed silicic acid ester preparations are used as binding agents for zinc dust pigments, pigment zinc dust is added to the binding agent in such quantity that there is developed a pigment which can be applied to a sand-blasted iron or steel surface by spraying, brushing or rolling. The coatings made of such pigment are hard and adhere tightly to the substrate and offer lasting protection from corrosion.
The technical advantages which the hydrolysis preparations of silicic acid esters offer are offset by their limited shelf life. During storage, hydrolyzed silicic acid esters are subject to alterations which limit the length of time for which they can be used. These alterations are based upon the fact that the free hydroxyl groups of the silanols are decomposed according to the equation: ##EQU2## In this process long-chained or branched siloxanes of higher molecular weight averaging more than 10 Si-O units are formed which have very few if any free hydroxyl groups. Such siloxanes therefore no longer have any binding properties and accordingly may no longer be used for the above-described applications. Since this siloxane formation may be considered to be an aging process, one therefore speaks of the "shelf life" of the hydrolyzates. This shelf life is dependent upon several factors, some of which are named herewith:
a. The amount of water used in the hydrolysis, PA1 b. The nature and amount of the catalyst, PA1 c. Temperature during performance of the hydrolysis, PA1 d. Duration of the hydrolysis reaction, PA1 e. Nature and amount of the solvent used, PA1 f. The influence of ultrasound during hydrolysis, and PA1 g. Temperature and atmospheric humidity at which the hydrolyzate is stored.
Heretofore, the lengthening of the shelf life of silicic acid ester hydrolyzates has been possible only by suitably balancing the above factors against one another. Therefore, it has become desirable to provide a means for lengthening the shelf life of such silicic acid ester hydrolyzates which does not involve balancing the above factors against one another. Such a method should be one which can be readily accomplished and which substantially lengthens the shelf life at least about 20%.